Generally, transmittance of plastics is very low for lights having a wavelength less than 300 nanometers, and therefore plastics cannot be used for such lights. On the other hand, deterioration of plastics due to lights having a wave length greater than 450 nanometers is negligible. Accordingly, a resistance of plastics against lights having a wavelength from 300 nanometers to 450 nanometers is significant. In the present text of specification, lights having a wavelength from 300 nanometers to 450 nanometers are also referred to as lights in the ultraviolet region including blue lasers.
Thus, plastics are likely to be damaged by lights having a wavelength from 300 nanometers to 450 nanometers. This means that they have a low resistance for such lights. Accordingly, in optical devices used with lights in the ultraviolet region including blue lasers, optical elements made of glass are mainly used.
On the other hand, a multi-layered optical film is often formed on a surface of an optical element of optical devices for the purpose of anti-reflection or the like. A method for producing an optical element provided with a multi-layered optical film which imparts to the element a resistance against lights in the ultraviolet region including blue lasers has been developed (JP4178190). However, a multi-layered optical film which imparts to an optical element a high resistance against lights in the ultraviolet region including blue lasers even in a high ambient temperature has not been developed.
Accordingly, there is a need for a multi-layered optical film which imparts to an optical element a high resistance against lights in the ultraviolet region including blue lasers even in a high ambient temperature.